The present invention relates to an attracting and fixing device for attracting and fixing a silicon wafer to a work head by air sucking in.
The surface of the silicon wafer on which an IC circuit is made is precisely ground, cut or measured with accuracy in the unit of .mu.m. In this case the silicon wafer is attracted and fixed on the work head by air sucking action. As the fixing method, methods shown in FIGS. 1 and 2 have been conventionally employed, wherein many circular slots in concentric circles are formed on the upper surface of the working base (1). The circular slots are connected with radial connecting slots. The silicon wafer (5) is sucked from an air conditioning hole (4) at the center for fixing it as shown in FIG. 1, or as the work head, an attracting disc of porous substance (6) is utilized. The periphery of it is surrounded with a base seat (7) of metal or synthetic resin with no permeability. Air sucking is performed through the air conducting hole (4) at the center for fixing the wafer as shown in FIG. 2.
According to the method shown in FIG. 1, an air gap is caused between the silicon wafer (5) and the attracting surface due to slots (2), (3).
If sucked heavily or pressed from the above when the wafer is ground and worked, the wafer is distorted locally causing error in cut size, or it becomes uneven when air is sucked out after working.
For example, when grinding a thin wafer of 0.1-0.5 mm (0.1 to 0.5 mm) thick the wafer is sucked heavily and pressed down by a rotary grindstone (8) from above. Therefore, it is curved downward at portions of slots (2), (3). If the wafer is so pressed down during grinding, when the pressure is released, only the portions facing to the slots expand.
This cannot be neglected in the wafer grinding in which an accuracy within .+-.5 .mu.m is required.
In a method shown in FIG. 2, the porous attracting disc (6) is composed of a substance having large pores for passing air and cleaning water. Therefore it has defects that it is clogged by dust of the grindstone, causing insufficient sucking and cleaning.
Further, in FIG. 2 the porous attracting disc (6) is necessarily surrounded by the base seat (7) made of material with no porosity.
If the base seat (7) is of metal, as the substance composing the attracting disc (6) and that composing the base seat are different from each other, the attracting disc (6) is probably distorted or peeled off from the base seat (7). Furthermore the method shown in FIG. 2 is defective in that, when the attracting disc (6) is ground, the upper layer of the base seat (7) is also ground, therefore the grindstone and the edged tool are damaged.
In the case that the base seat is made of synthetic resin, the grindstone and the edged tool are not damaged at the time of grinding, however, when the attracting disc (6) is joined with the base seat (7), it is likely that the adhesive resin comes up to the attracting surface through the interior of the porous attracting disc (6), or the attracting surface becomes uneven or distorted due to the difference of the expansion coefficient between the attracting disc and the base seat.